Summary - Groundwater Vulnerability: Chernobyl Nuclear Disaster - page 102

and southwestern parts, respectively, of the Kyiv region and to separate clean
spots. The majority of the study area is characterized by average groundwater
vulnerability with equivalent 137 Cs surface contamination densities from 0.01 to
0.1 Ci/km 2 and concentration in groundwater within 1-10 mBq/dm 3 .
The assessment of vulnerability and protectability of the second (from the
surface) Eocene aquifer was performed using the developed methodology with
account of data of independent groundwater flow modeling with a four-layer
hydrogeological model of the study area. The model provided the necessary data
regarding vertical flow through the confining low-permeability bed of Kyiv marls.
Consequently, the assessment was performed in such a way as to implicitly
account for the transit flow into the underlying deeper aquifers (Cenomanian-
Callovian and Bajocean), which are intensively exploited by the groundwater
intakes for the water supply of Kyiv City. The obtained map of the Eocene aquifer
protectability to contamination with Chernobyl-born 137 Cs (Figure 5.10)
represents the areal distribution of the groundwater protectability index ε = - log
c (where c is dimensionless contaminant concentration relative to the initial
surface) with gradations of its value from 1 to 10 and corresponding gradations
of relative concentration in percent. The map accounts for the total hydraulic and
geochemical attenuation capacity of the upper Quaternary aquifer and the con-
fining bed of Kyiv marls. Characteristic zones of relatively weak aquifer protect-
ability correspond to areas of no or weak occurrence of the confining bed and its
disturbances (PFMZs), which in turn are characterized by significant downward
flow velocities. Depression cones from the operating water intakes of Kyiv City
and Pripyat town (within CEZ) in the underlying Cenomanian-Callovian and
Bajocean aquifers also have a noticeable influence on the formation of these weak
protectability zones of the Eocene aquifer. These depression cones form increased
downward flow components in the upper aquifer and the first confining bed.
Using the results of modeling of predicted groundwater contamination
with 137 Cs of the upper Quaternary aquifer, the vulnerability map of the
Eocene aquifer has been drawn in units of surface contamination density
(kBq/m 2 and Ci/km 2 ; Figure 5.11). This was done by “overlying” the upper
aquifer vulnerability map (Figure 5.5) onto the working map of the total
attenuation capacity of the saturated part of the upper Quaternary aquifer
and the Kyiv marl confining bed. The approximate assessment of the Eocene
aquifer vulnerability to 137 Cs is performed in units of its predicted concentration
in groundwater (mBq/dm 3 ). This was done by the recalculation of the total
assessed cover contamination (in kBq/m 2 ) infiltrating into the aquifer in to the
concentration in groundwater (mBq/dm 3 ) with account of the concentration
decrease caused by sorption of contamination by aquifer rock calculated using
data of aquifer thickness, porosity, rock density, and distribution coefficient
K d . The zones of highest Eocene aquifer vulnerability correspond to areas of
highest surface contamination within the CEZ. However, their area is restricted
within sites of relatively low aquifer protectability (Figure 5.10). On the whole,